mm: compaction: capture a suitable high-order page immediately when it is made available

mm: compaction: capture a suitable high-order page immediately when it is made available

While compaction is migrating pages to free up large contiguous blocks for
allocation it races with other allocation requests that may steal these
blocks or break them up.  This patch alters direct compaction to capture a
suitable free page as soon as it becomes available to reduce this race.
It uses similar logic to split_free_page() to ensure that watermarks are
still obeyed.

Signed-off-by: Mel Gorman <mgorman@suse.de>
Reviewed-by: Rik van Riel <riel@redhat.com>
Reviewed-by: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>